A thin-film transistor (TFT) is a special kind of field-effect transistor (FET) made by depositing thin films of an active semiconductor layer, as well as the dielectric layer and metallic contacts, over a supporting, but non-conducting substrate. A common substrate is glass, because the primary application of TFTs is in liquid-crystal displays (LCDs). This differs from conventional transistors, in which the semiconductor material (e.g., silicon) typically is the substrate.
TFTs can be made using a wide variety of semiconductor materials, such as silicon (Si). The characteristics of a silicon-based TFT depend on the silicon's crystalline state; that is, the semiconductor layer can be either amorphous silicon, microcrystalline silicon, or silicon annealed into polysilicon.
Other materials which have been used as semiconductors in TFTs include compound semiconductors, such as cadmium selenide, or metal oxides, such as zinc oxide or hafnium oxide. An application for hafnium oxide is as a high dielectric constant (high-κ) material. TFTs have also been made using organic materials, referred to as organic field-effect transistors (OTFTs).
By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent. Such transparent TFTs (TTFTs) can be used for construction of video display panels. Because conventional substrates cannot withstand high annealing temperatures, the deposition process may most likely be completed under relatively low temperatures. Chemical vapor deposition and physical vapor deposition (usually sputtering) may be utilized.